1. Field of the Invention
This invention relates to an optical system driving degree adjusting device for the photo taking lens or the like of a camera, and more particularly to a novel optical system adjustment device for a camera which is capable of promptly adjusting the focal point and varying the magnification of the optical system with a high degree of precision.
2. Description of the Related Art
Heretofore many and various contrivances have been proposed for adjustment of the image forming optical systems of cameras, such as an automatic focusing (hereinafter referred to AF) device.
The AF devices employed in the conventional commercialized cameras and those disclosed in the proposals made in the past are arranged to shift the position of a lens barrel in the direction of its optical axis by means of a screw feed device and to bring the lens barrel to a stop in a given position by allowing a lock claw to plung into lock teeth arranged in a saw-teeth like shape in one body with the lens barrel. A single pitch thread is formed on the screw feed device. The lock claw is under the digital control of an electronic control circuit.
The AF device of this kind must be arranged to be capable of highly precisely stopping the lens barrel at a predetermined stop position for a sufficiently high degree of focusing precision. To meet this requirement, it has been necessary to adopt various measures including, among others, finely dividing the pitch of the thread of the screw feed device and that of the lock teeth.
However, the effect attainable by the above stated measures is limited and has failed to enable the conventional AF device to perform a focusing action with a high speed and a high degree of precision because of the following reasons:
(a) The arrangement to have a finer pitch of the screw feed device naturally lowers the speed at which the lens barrel can be shifted and makes a quick focusing action impossible. To solve this problem, the screw feed device must be rotated at a higher speed. However, an increased number of gears lowers the efficiency of a force transmission system. Besides, the backlash of these gears lowers the lens barrel shifting and stopping precision. Also, the smaller pitch lowers the strength of the screw thread and increases an abrasion loss.
(b) The lens barrel positioning accuracy increases as the pitch of the lock teeth decreases. However, the teeth become smaller to have lower mechanical strength as the pitch of the teeth decreases. The pitch of the teeth has a reducible limit in terms of strength. The reduction in the lock teeth pitch is further limited by the plunging time of the lock claw.
According to the normally available strength and energy of parts usable for a camera, the plunging time allowable to the lock claw in plunging into the lock teeth is from 2 to 4 ms. The lock claw passing time for passing each of the lock teeth (equal to one pitch at which the lens barrel is to be drawn forward) on the other hand must be arranged to be sufficiently longer than 4 ms. Therefore, assuming that the lock claw passing time of each of the lock teeth is 10 ms and that the object distance is divided into 100 zones (i.e. a maximum lens barrel shifting distance is 100 zones), a length of time of 10 ms.times.100 zones=1 sec is required in shifting the lens barrel for focusing to an extent corresponding to this object distance. Therefore, the camera provided with the conventional AF device has a maximum release time lag as much as one sec before completion of a focusing action after a shutter release button is pushed. This time lag is too long for seizing a good shutter chance.